Frequency-hopping control method and module, and DC/DC converter

ABSTRACT

A frequency-hopping control method is performed by a frequency-hopping control module that generates a driving signal for driving a voltage converting circuit to generate an output voltage. The method includes, generating a control signal according to a regulating signal inversely proportional to the output voltage of the voltage converting circuit. The control signal is cyclical, and each cycle of which includes an off-time having a variable duration with an inverse relation to magnitude of the regulating signal, and an on-time having a substantially fixed duration. The driving signal is generated according to the control signal and a periodic pulse signal. Therefore, the output voltage can be stabilized, and the voltage converting circuit can perform voltage conversion with reduced power loss and improved voltage conversion efficiency.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Chinese Application No.200910041634.0, filed on Jul. 30, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage-conversion control method,more particularly to a frequency-hopping control method for a voltageconverting circuit that operates in a frequency-hopping mode.

2. Description of the Related Art

Referring to FIG. 1, a conventional DC/DC converting device 900 includesa DC/DC converter 901 and a feedback control circuit 902. The DC/DCconverter 901 is a half-bridge LLC oscillating circuit. The feedbackcontrol circuit 902 includes an ST L6599 control chip 903.

As the load current decreases (as a result of increasing load resistanceR_(o)), the output voltage V_(o) of the DC/DC converter 901 increasesslightly, causing the voltage at node A, V_(comp), to drop. WhenV_(comp) drops below a preset voltage of the control chip 903, thefeedback control circuit 902 stops generating driving signals HVG andLVG, which drive the DC/DC converter 901, such that the DC/DC converter901 stops voltage conversion operation thereof. During an off-time ofthe DC/DC converter 901, the output voltage V_(o) of the DC/DC converter901 drops slightly, causing V_(comp) to rise. Once V_(comp) reaches apreset voltage, the feedback control circuit 902 resumes output of thedriving signals HVG and LVG, and the DC/DC converter 901 transitions toan on-time.

Therefore, the conventional feedback control circuit 902 controls theoperating state of the DC/DC converter 901 according to V_(comp).Further referring to FIG. 2, V_(comp) is a gradually-changing voltage.That is to say, during each on-time, duration of V_(comp) dropping tothe preset voltage of the control chip 903 (see duration marked byt91-t92) is too long in relation to the change in the output voltageV_(o). As a result, the DC/DC converter 901 operates for longer than anoptimized on-time. As shown by the inductor current I_(pri) in FIG. 2,the cycles of I_(pri) during the duration marked by t91-t92 in theon-time convert only a small proportion of the input energy. Therefore,the efficiency of the conventional DC/DC converting device 900, in termsof power loss, deteriorates if the on-time is too long.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide afrequency-hopping control method capable of reducing power consumption.

Accordingly, the frequency-hopping control method of the presentinvention is to be performed by a frequency-hopping control module thatis suitable for use with a voltage converting circuit and that generatesa driving signal for driving the voltage converting circuit when thevoltage converting circuit operates in a frequency-hopping mode suchthat the voltage converting circuit generates an output voltage. Thefrequency-hopping control method includes: a first step of generating aperiodic pulse signal; a second step of generating a control signalaccording to a regulating signal that is inversely proportional to theoutput voltage of the voltage converting circuit, wherein control signalis cyclical and each cycle of the control signal includes an off-timeand an on-time, the on-time having a substantially fixed duration, theoff-time having a variable duration that has an inverse relation tomagnitude of the regulating signal; and a third step of generating adriving signal according to the control signal and the pulse signal fordriving the voltage-converting circuit.

Preferably, in the second step, the regulating signal is for controllingan output current of a voltage-controlled current source, and theduration of the off-time corresponds to duration of charging of astorage capacitor to a first threshold voltage using the output currentof the voltage-controlled current source.

Preferably, in the second step, the duration of the on-time correspondsto duration of discharging of the storage capacitor via a resistor to asecond threshold voltage lower than the first threshold voltage.

Preferably, in the third step, the driving signal is generated byperforming a logic AND operation on the control signal and the pulsesignal.

Another object of the present invention is to provide afrequency-hopping control module capable of reducing power consumption.

Accordingly, a frequency-hopping control module of the present inventionis suitable for use with a voltage converting circuit and is capable ofgenerating a driving signal for driving the voltage converting circuitwhen the voltage converting circuit operates in a frequency-hopping modesuch that the voltage converting circuit generates an output voltage.The frequency-hopping control module includes a high-frequency signalgenerator, an off-time regulator, and a driving signal-generatingcircuit. The high-frequency signal generator is for generating aperiodic pulse signal. The off-time regulator is for generating acontrol signal according to a regulating signal that is inverselyproportional to the output voltage of the voltage converting circuit.The control signal is cyclical and each cycle of the control signalincludes an off-time and an on-time. The on-time has a substantiallyfixed duration, and the off-time has a variable duration that has aninverse relation to magnitude of the regulating signal. The drivingsignal-generating circuit is coupled to the high-frequency signalgenerator and the off-time regulator for generating the driving signalaccording to the control signal and the pulse signal.

Preferably, the off-time regulator includes a voltage-controlled currentsource, a storage capacitor, and a hysteretic comparator. Thevoltage-controlled current source is for generating an output currentaccording to the regulating signal. The storage capacitor has a firstterminal coupled to the voltage-controlled current source, and agrounded second terminal. The hysteretic comparator has a non-invertingterminal coupled to the first terminal of the storage capacitor, aninverting terminal to receive a reference voltage, and an outputterminal. The hysteretic comparator defines a hysteretic zone accordingto the reference voltage, and the hysteretic zone defines a firstthreshold voltage. The hysteretic comparator limits duration of chargingof the storage capacitor using the output current of thevoltage-controlled current source to the first threshold voltage so asto define the duration of the off-time. The control signal of theoff-time regulator is outputted from the output terminal of thehysteretic comparator.

Preferably, the off-time regulator further includes a resistor having afirst terminal coupled to the first terminal of the storage capacitor,and a grounded second terminal. The hysteretic zone further defines asecond threshold voltage lower than the first threshold voltage. Thehysteretic comparator limits duration of discharging of the storagecapacitor via the resistor to the second threshold voltage so as todefine the duration of the on-time.

Preferably, the off-time regulator further includes a first switch, asecond switch, and an inverter. The first switch is connected in seriesbetween the voltage-controlled current source and the storage capacitor.The second switch is connected in series between the resistor andground. The inverter has an input terminal coupled to the outputterminal of the hysteretic comparator, and an output terminal coupled tothe first switch for controlling opening and closing of the firstswitch. The output terminal of the hysteretic comparator is coupled tothe second switch for controlling opening and closing of the secondswitch. The first and second switches cooperate to switch the storagecapacitor between charging and discharging states.

Yet another object of the present invention is to provide a DC/DCconverting device capable of reducing power consumption.

Accordingly, a DC/DC converting device of the present invention includesa voltage converting circuit and a frequency-hopping control module. Thevoltage converting circuit is operable in a frequency-hopping mode. Thefrequency-hopping control module includes a high-frequency signalgenerator, an off-time regulator, and a driving signal-generatingcircuit, details of which are the same as those described hereinabove.

The frequency-hopping control module of the present invention is capableof regulating duration of the off-time of the control signal accordingto the output voltage of the voltage-converting circuit, therebyensuring that the on-time of the control signal has an optimum duration.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent in the following detailed description of the preferredembodiment with reference to the accompanying drawings, of which:

FIG. 1 is a schematic circuit diagram illustrating internal componentsof a conventional DC/DC converting device;

FIG. 2 is a timing diagram to illustrate waveforms of HVG, LVG,V_(comp), and I_(pri) signals in the conventional DC/DC convertingdevice;

FIG. 3 is a schematic circuit diagram of a DC/DC converting device ofthe preferred embodiment of the present invention;

FIG. 4 is a schematic circuit diagram of an off-time regulator of afrequency-hopping control module of the DC/DC converting device of thepreferred embodiment;

FIG. 5 illustrates a hysteresis zone defined by a hysteretic comparatorof the off-time regulator of the preferred embodiment;

FIG. 6 is a timing diagram illustrating how the off-time regulatorgenerates a control signal LF;

FIG. 7 is a flow chart illustrating consecutive steps of thefrequency-hopping control method of the present invention; and

FIG. 8 is a timing diagram to illustrate waveforms of Drv01, Drv02, LF,Drv1, Drv2, V_(o), V_(reg), and I_(pri) signals in the frequency-hoppingcontrol module of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 3, a DC/DC converting device 100 of the preferredembodiment according to the present invention employs theFrequency-Hopping Spread Spectrum (FHSS) technique to regulate durationof an off-time of a voltage-converting circuit 1 while maintainingduration of an on-time of the voltage-converting circuit 1 constant,thereby transferring energy at a high efficiency and reducing power lossunder light load conditions. In the present embodiment, the DC/DCconverting device 100 includes the voltage converting circuit 1 and afrequency-hopping control module 2.

The voltage converting circuit 1 can be one of isolated and non-isolatedDC/DC converters, such as a buck converter, a boost converter, abuck-boost converter, a flyback converter, a forward converter, and anLLC oscillating circuit. The voltage converting circuit 1 of thepreferred embodiment is a half-bridge LLC oscillating circuit and has afirst power switch Q1 and a second power switch Q2.

The frequency-hopping control module 2 is for generating driving signalsto control the first and second power switches Q1,Q2 of the voltageconverting circuit 1. In the present embodiment, the voltage convertingcircuit 1 operates in a frequency-hopping mode in which thefrequency-hopping control module 2 switches the voltage convertingcircuit 1 between an on-time and an off-time. During the on-time, thefrequency-hopping control module 2 outputs the driving signals fordriving the voltage converting circuit 1 so that the voltage convertingcircuit 1 performs voltage conversion. Conversely, the frequency-hoppingcontrol, module 2 opens (i.e., turns off) the first and second powerswitches Q1, Q2 during the off-time so that the voltage convertingcircuit 1 does not perform voltage conversion. The frequency-hoppingcontrol module 2 includes a feedback regulator 3, a high-frequencysignal generator 4, an off-time regulator 5, and a drivingsignal-generating circuit 6.

The feedback regulator 3 is for generating a regulating signal V_(reg)according to an output voltage V_(o) of the voltage converting circuit1. The regulating signal V_(reg) is for generating a control signal LFand for controlling switching frequency of the driving signals generatedby the driving signal-generating circuit 6. The feedback regulator 3transmits the regulating signal V_(reg) to the high-frequency signalgenerator 4, which generates a set of periodic pulse signalsDrv01,Drv02, and to the off-time regulator 5, which generates thecontrol signal L_(F) according to the regulating signal V_(reg). Furtherdetails of the high-frequency signal generator 4 and the off-timeregulator 5 will be described later in the specification. The drivingsignal-generating circuit 6 generates the driving signals Drv1,Drv2 fordriving the first and second power switches Q1,Q2 according to the pulsesignals Drv01,Drv02 and the control signal LF. Preferably, a frequencyof the pulse signals Drv01, Drv02 is a maximum allowable switchingfrequency of the first and second power switches Q1, Q2 of the voltageconverting circuit 1.

It is to be noted that, when the load current decreases (due toincreasing load resistance R_(o)), the output voltage V_(o) of thevoltage converting circuit 1 increases slightly, and switching frequencyof the first and second power switches Q1,Q2 needs to be increased so asto stabilize the output voltage V_(o). Therefore, the voltage of theregulating signal V_(reg) needs to be decreased. In other words, theoutput voltage V_(o) is inversely proportional to the regulating signalV_(reg).

Referring to FIG. 4, the off-time regulator 5 of includes avoltage-controlled current source 51, a first switch S1, a second switchS2, a storage capacitor C, a resistor R, an inverter 52, and ahysteretic comparator 53.

The voltage-controlled current source 51 adjusts a charging currentoutputted thereby according to the regulating signal V_(reg). The firstswitch S1 has a first terminal 501 coupled to the voltage-controlledcurrent source 51, and a second terminal 502. The storage capacitor Chas a first terminal 503 coupled to the second terminal 502 of the firstswitch S1, and a grounded second terminal 504. The resistor R has afirst terminal 505 coupled to the first terminal 503 of the storagecapacitor C, and a second terminal 506. The second switch S2 has a firstterminal 507 coupled to the second terminal 506 of the resistor R, and agrounded second terminal 508. The hysteretic comparator 53 has anon-inverting terminal coupled to the first terminal 503 of the storagecapacitor C, an inverting terminal to receive a reference voltageV_(ref), and an output terminal. The input terminal of the inverter 52is coupled to the output terminal of the hysteretic comparator 53, andthe output terminal of the inverter 52 is coupled to the first switch S1for controlling opening and closing of the first switch S1. The outputterminal of the hysteretic comparator 53 is further coupled to thesecond switch S2 for controlling opening and closing of the secondswitch S2.

It is to be noted that the hysteretic comparator 53 defines a hysteresiszone according to the reference voltage V_(reg). Referring to FIG. 5,the hysteresis zone defines a first threshold voltage V_(H) and a secondthreshold voltage V_(L) lower than the first threshold voltage V_(H). Inaddition, the control signal LF is the signal outputted at the outputterminal of the hysteretic comparator 53.

Referring to FIG. 6, initially, the control signal LF is at a low levelsuch that the first switch S1 is closed (i.e., on) and the second switchS2 is open (i.e., off), and the initial voltage V_(c) of the storagecapacitor C is zero. Therefore, upon receiving the regulating signalV_(reg), the voltage-controlled current source 51 generates acorresponding charging current that charges the storage capacitor C(duration between t₀-t₁). When the voltage V_(c) of the storagecapacitor C is charged to the first threshold voltage L₁ (V_(H)), thecontrol signal LF switches to a high level so that the first switch isopen and that charging of the storage capacitor C stops.

It is to be noted that the control signal LF is cyclical, and each cycleof the control signal LF includes an on-time and an off-time. Durationof charging of the storage capacitor C to the first threshold voltage L₁(V_(H)) is equivalent to duration of the off-time of the control signalLF. The control signal LF is at the low level while the storagecapacitor C is charging. In other words, the driving signals Drv1 andDrv2 generated by the driving signal-generating circuit 6 have anon-time and an off-time that correspond to the on-time and the off-timeof the control signal LF, as best shown in FIG. 8. The voltageconverting circuit 1 operates during the on-time, but not during theoff-time.

When the control signal LF switches to the high level, the first switchS1 is open (i.e., off) and the second switch S2 is closed (i.e., on),and the storage capacitor C starts to discharge via the resistor R(duration between t₁-t₂ in FIG. 6). It is to be noted that duration ofdischarging of the storage capacitor C is determined by the RC timeconstant. When the voltage V_(e) of the storage capacitor C decreases toand drops below the second threshold voltage L2 (V_(L)) defined by thehysteretic comparator 53 during discharging, and the control signal LFis switched to the low level.

In the present embodiment, duration of the on-time of the control signalLF is duration of discharging of the storage capacitor C to the secondthreshold voltage L2 (V_(L)). During the on-time, the control signal LFis at the high level. Since capacitance of the capacitor C andresistance of the resistor R are fixed, duration of discharging of thestorage capacitor C is equal to the RC time constant. That is to say,the on-time of the control signal is a fixed duration of time. In otherwords, duration of time interval t₁-t₂ is substantially equal to that oftime interval t₃-t₄. However, duration of time interval t₂-t₃ isdetermined by the regulating signal V_(reg): the smaller the regulatingsignal V_(reg), the longer the duration of charging of the capacitorvoltage V_(c) of the storage capacitor C to the first threshold voltageL1 (V_(H)), and hence the longer the duration of the off-time (e.g.,time interval t₀-t₁) of the control signal LF. Conversely, the largerthe regulating signal V_(reg), the shorter the duration of charging ofthe capacitor voltage V_(e) of the storage capacitor C to the firstthreshold voltage L1 (V_(H)), and hence the shorter the duration of theoff-time (e.g., time interval t₂-t₃).

Overall, under circumstances in which the load current decreases (due toincreasing load resistance R_(o)), the output voltage V_(o) of thevoltage converting circuit 1 increases, and the voltage of theregulating signal V_(reg) decreases (inversely proportional to eachother). Subsequently, the voltage-controlled current source 51 outputs asmaller charging current, the charging duration is lengthened, theoff-time of the control signal LF is lengthened, the voltage convertingcircuit 1 idles for longer duration, and the output voltage V_(o) drops.Conversely, under circumstances in which the output voltage V_(o) of thevoltage converting circuit 1 drops, the voltage of the regulating signalV_(reg) rises so that the voltage-controlled current source 51 cancharge the storage capacitor C with a larger current, thus shorteningduration of the off-time while duration of on-time remains unchanged.Therefore, within one operating cycle, the voltage converting circuit 1performs voltage conversion for a relatively longer on-time such thatthe output voltage V, rises. Thus, the frequency-hopping control module2 can stabilize the output voltage v, of the voltage converting circuit1.

The operation of the frequency-hopping control module 2 will now bedescribed with reference to FIGS. 3,7, and 8, in which FIG. 7 is a flowchart of the frequency-hopping control method of the present embodiment,and FIG. 8 is timing diagram to illustrate waveforms of signalsgenerated by the frequency-hopping control module 2.

In step 10, the feedback regulator 3 generates a regulating signalV_(reg) that is inversely proportional to the output voltage V_(o) ofthe voltage converting circuit 1, and provides the regulating signalV_(reg) to the high-frequency signal generator 4 and the off-timeregulator 5.

In step 20, the high-frequency signal generator 4 generates periodicpulse signals Drv01, Drv02 according to the regulating signal V_(reg).The pulse signals Drv01, Drv02 are provided to the drivingsignal-generating circuit 6.

While the high-frequency signal generator performs step 20, the off-timeregulator 5 performs step 30 and step 90 upon receiving the regulatingsignal V_(reg).

In step 30, output current of the voltage-controlled current source 51of the off-time regulator 5 is controlled by the regulating signalV_(reg), and is used to charge the storage capacitor C of the off-timeregulator 5 so as to regulate the off-time of the control signal LF,namely time interval t₂₀-t₃₀. Referring to FIG. 8, in time intervalt₂₀-t₃₀, the slope of the regulating signal V_(reg) is less steeper suchthat charging of the storage capacitor C to the first threshold voltageV_(H) is slower, and hence the off-time duration is longer. In contrast,in the time interval t₄₀-t₅₀, the slope of the regulating signal V_(reg)is relatively steeper such that charging of the storage capacitor C tothe first threshold voltage. V_(H) is faster, and hence the off-timeduration is shorter.

In step 40, the off-time regulator 5 controls the on-time of the controlsignal LF by using the RC time constant of discharging of the storagecapacitor C via the resistor R, and the hysteresis zone defined by thehysteretic comparator 53, namely, time interval t₁₀-t₂₀. In the presentembodiment, duration of the on-time is substantially equivalent toduration of three consecutive pulse cycles of either one of the pulsesignals Drv01, Drv02. In other words, the capacitance of the storagecapacitor C and the resistance of the resistor R need to be properlydesigned such that the RC time constant is substantially equivalent tothree pulse cycles of the pulse signals Drv01, Drv02.

In step 50, the driving signal-generating circuit 6 generates thedriving signals Drv1, Drv2 for driving the first and second powerswitches Q1, Q2 from the pulse signals Drv01, Drv02, and the controlsignal LF. In the present embodiment, the driving signal-generatingcircuit 6 is a digital logic circuit that performs a logic AND operationupon the pulse signals Drv01, Drv02 and the control signal LF togenerate the driving signals Drv1, Drv2.

Therefore, the driving signals Drv1, Drv2 generated by thedriving-signal generating circuit 6 control the opening and closing ofthe first and second power switches Q1, Q2 so that the storage inductorL_(r) of the voltage converting circuit 1 charges and discharges. Thewaveform of the inductor current I_(pri) is shown in FIG. 8.

It is to be noted that duration of the on-time of the control signal LFis not limited to three pulse cycles of either one of the pulse signalsDrv01, Drv02. Instead, duration of the on-time can be changed accordingto need by appropriately designing resistance of the resistor R andcapacitance of the capacitor C, or the hysteresis zone of the hystereticcomparator 53. It is also to be noted that only duration of the off-timeis regulated, and that duration of the on-time is fixed, while thefrequency-hopping control module 2 operates.

In summary, the frequency-hopping control module 2 of the DC/DCconverting device 100 of the present invention generates driving signalsDrv1, Drv2 each having an on-time with a substantially fixed durationand an off-time with a variable duration such that the output voltageV_(o) can be stabilized, and that the voltage converting circuit 1 canperform voltage conversion for a preset duration of the on-time so as toreduce power loss and improve voltage conversion efficiency.

While the present invention has been described in connection with whatis considered the most practical and preferred embodiment, it isunderstood that this invention is not limited to the disclosedembodiment but is intended to cover various arrangements included withinthe spirit and scope of the broadest interpretation so as to encompassall such modifications and equivalent arrangements.

1. A frequency-hopping control method to be performed by afrequency-hopping control module that is suitable for use with a voltageconverting circuit and that generates a driving signal for driving thevoltage converting circuit when the voltage converting circuit operatesin a frequency-hopping mode such that the voltage converting circuitgenerates an output voltage, comprising the steps of: (A) generating aperiodic pulse signal; (B) generating a control signal according to aregulating signal that is inversely proportional to the output voltageof the voltage converting circuit, wherein the control signal iscyclical and each cycle of the control signal includes an off-time andan on-time, the on-time having a substantially fixed duration, theoff-time having a variable duration that has an inverse relation tomagnitude of the regulating signal; and (C) generating the drivingsignal according to the control signal and the pulse signal for drivingthe voltage converting circuit.
 2. The frequency-hopping control methodas claimed in claim 1, wherein, in step A), a frequency of the periodicpulse signal is a maximum allowable switching frequency of the voltageconverting circuit.
 3. The frequency-hopping control method as claimedin claim 1, wherein, in step B), the regulating signal is used tocontrol an output current of a voltage-controlled current source, andthe duration of the off-time corresponds to duration of charging of astorage capacitor to a first threshold voltage using the output currentof the voltage-controlled current source.
 4. The frequency-hoppingcontrol method as claimed in claim 3, wherein, in step B), the durationof the on-time corresponds to duration of discharging of the storagecapacitor via a resistor to a second threshold voltage lower than thefirst threshold voltage.
 5. The frequency-hopping control method asclaimed in claim 1, wherein, in step C), the driving signal is generatedby performing a logic AND operation upon the control signal and thepulse signal.
 6. A frequency-hopping control module suitable for usewith a voltage converting circuit and capable of generating a drivingsignal for driving the voltage converting circuit when the voltageconverting circuit operates in a frequency-hopping mode such that thevoltage converting circuit generates an output voltage, saidfrequency-hopping control module comprising: a high-frequency signalgenerator for generating a periodic pulse signal; an off-time regulatorfor generating a control signal according to a regulating signal that isinversely proportional to the output voltage of the voltage convertingcircuit, wherein the control signal is cyclical and each cycle of thecontrol signal includes an off-time and an on-time, the on-time having asubstantially fixed duration, the off-time having a variable durationthat has an inverse relation to magnitude of the regulating signal; anda driving signal-generating circuit coupled to said high-frequencysignal generator and said off-time regulator for generating the drivingsignal according to the control signal and the pulse signal.
 7. Thefrequency-hopping control module as claimed in claim 6, wherein saidoff-time regulator includes a voltage-controlled current source forgenerating an output current according to the regulating signal, astorage capacitor having a first terminal coupled to saidvoltage-controlled current source, and a grounded second terminal, and ahysteretic comparator having a non-inverting terminal coupled to saidfirst terminal of said storage capacitor, an inverting terminal toreceive a reference voltage, and an output terminal, said hystereticcomparator defining a hysteretic zone according to the referencevoltage, the hysteretic zone defining a first threshold voltage, saidhysteretic comparator limiting duration of charging of said storagecapacitor using the output current of said voltage-controlled currentsource to the first threshold voltage so as to define the duration ofthe off-time, the control signal of said off-time regulator beingoutputted from said output terminal of said hysteretic comparator. 8.The frequency-hopping control module as claimed in claim 7, wherein saidoff-time regulator further includes a resistor having a first terminalcoupled to said first terminal of said storage capacitor, and a groundedsecond terminal, the hysteretic zone further defining a second thresholdvoltage lower than the first threshold voltage, said hystereticcomparator limiting duration of discharging of said storage capacitorvia said resistor to the second threshold voltage so as to define theduration of the on-time.
 9. The frequency-hopping control module asclaimed in claim 8, wherein said off-time regulator further includes afirst switch connected in series between said voltage-controlled currentsource and said storage capacitor, a second switch connected in seriesbetween said resistor and ground, and an inverter having an inputterminal coupled to said output terminal of said hysteretic comparator,and an output terminal coupled to said first switch for controllingopening and closing of said first switch, said output terminal of saidhysteretic comparator being coupled to said second switch forcontrolling opening and closing of said second switch, said first andsecond switches cooperating to switch said storage capacitor betweencharging and discharging states.
 10. The frequency-hopping controlmodule as claimed in claim 6, further comprising a feedback regulatorfor generating the regulating signal according to the output voltage ofthe voltage converting circuit.
 11. The frequency-hopping control moduleas claimed in claim 6, wherein a frequency of the periodic pulse signalgenerated by said high-frequency signal generator is a maximum allowableswitching frequency of the voltage converting circuit.
 12. Thefrequency-hopping control module as claimed in claim 6, wherein saiddriving signal-generating circuit performs a logic AND operation uponthe control signal and the pulse signal to generate the driving signal.13. A DC/DC converter comprising: a voltage converting circuit operablein a frequency-hopping mode to generate an output voltage; and afrequency-hopping control module including a high-frequency signalgenerator for generating a periodic pulse signal, an off-time regulatorfor generating a control signal according to a regulating signal that isinversely proportional to the output voltage of the voltage convertingcircuit, wherein the control signal is cyclical and each cycle of thecontrol signal includes an off-time and an on-time, the on-time having asubstantially fixed duration, the off-time having a variable durationthat has an inverse relation to magnitude of the regulating signal, anda driving signal-generating circuit coupled to said high-frequencysignal generator, said off-time regulator and said voltage convertingcircuit for generating a driving signal for driving said voltageconverting circuit according to the control signal and the pulse signal.14. The DC/DC converter as claimed in claim 13, wherein said off-timeregulator includes a voltage-controlled current source for generating anoutput current according to the regulating signal, a storage capacitorhaving a first terminal coupled to said voltage-controlled currentsource, and a grounded second terminal, and a hysteretic comparatorhaving a non-inverting terminal coupled to said first terminal of saidstorage capacitor, an inverting terminal to receive a reference voltage,and an output terminal, said hysteretic comparator defining a hystereticzone according to the reference voltage, the hysteretic zone defining afirst threshold voltage, said hysteretic comparator limiting duration ofcharging of said storage capacitor using the output current of saidvoltage-controlled current source to the first threshold voltage so asto define the duration of the off-time, the control signal of saidoff-time regulator being outputted from said output terminal of saidhysteretic comparator.
 15. The DC/DC converter as claimed in claim 14,wherein said off-time regulator further includes a resistor having afirst terminal coupled to said first terminal of said storage capacitor,and a grounded second terminal, the hysteretic zone further defining asecond threshold voltage lower than the first threshold voltage, saidhysteretic comparator limiting duration of discharging of said storagecapacitor via said resistor to the second threshold voltage so as todefine the duration of the on-time.
 16. The DC/DC converter as claimedin claim 15, wherein said off-time regulator further includes a firstswitch connected in series between said voltage-controlled currentsource and said storage capacitor, a second switch connected in seriesbetween said resistor and ground, and an inverter having an inputterminal coupled to said output terminal of said hysteretic comparator,and an output terminal coupled to said first switch for controllingopening and closing of said first switch, said output terminal of saidhysteretic comparator being coupled to said second switch forcontrolling opening and closing of said second switch, said first andsecond switches cooperating to switch said storage capacitor betweencharging and discharging states.
 17. The DC/DC converter as claimed inclaim 13, wherein said frequency-hopping control module further includesa feedback regulator coupled to said voltage converting circuit and saidoff-time regulator for generating the regulating signal according to theoutput voltage of said voltage converting circuit.
 18. The DC/DCconverter as claimed in claim 13, wherein a frequency of the periodicpulse signal generated by said high-frequency signal generator is amaximum allowable switching frequency of said voltage convertingcircuit.
 19. The DC/DC converter as claimed in claim 13, wherein saiddriving signal-generating circuit performs a logic AND operation uponthe control signal and the pulse signal to generate the driving signal.20. The DC/DC converter as claimed in claim 13, wherein said voltageconverting circuit is a half-bridge LLC oscillating circuit.